Heretofore, resin compositions comprising polyphenylene ether resins and rubber-reinforced styrenic resins have been used widely for uses such as industrial parts, electrical parts, office instrument housing, automobile parts, precision parts, etc., by utilizing various characteristics such as moldability, workability, heat resistance, impact resistance, flame retardancy, electrical characteristics and others, and their amounts used are markedly increasing in recent years. With such an increase of the amount used, improvements of the resin characteristics are strongly desired.
For instance, in a molded product having a large scale and a complicated structure, it is required to have high impact strength at the position where molding strain tends to remain, high impact strength at the broad flat portion and uniform luster over the whole of the molded product. Further, some molded products may have metals inserted therein, whereby crack resistance is greatly demanded. Furthermore, there is an increasing tendency to employ an injection molding system by a pin gate or hot runner with an aim to make the design of the molded product higher or power saving, and for this purpose it is also required to generate no silver streaks under high temperature. That is, to comply with diversification of uses in the future, it is indispensable that the overall balance of all the characteristics should be good. However, such a requirement cannot at all be accomplished in the conventional resin compositions comprising polyphenylene ether resins and rubber-reinforced styrenic resins. To cope with such problems, the following techniques have been proposed.
In a resin composition comprising a polyphenylene ether resin and a rubber-reinforced styrenic resin, as can be seen in U.S. Pat. No. 4,128,602, when the rubber particle size is merely made smaller, dart impact strength and crack resistance are inferior, although Izod impact strength and luster are excellent. On the contrary, when the rubber particle size is made greater, Izod impact strength and luster are inferior, although dart impact strength and crack resistance are good. Also, according to experiences, when there exists a rubber having large particle sizes, the generation of silver streaks has been the problem. On the other hand, as can be seen in U.S. Pat. No. 4,436,854, by merely making rubber particles having small particle sizes and rubber particles having large particle sizes to exist, the balance of the practical characteristics such as impact strength, luster, crack resistance cannot be satisfactorily maintained, and further it is entirely impossible to cancel the generation of silver streaks having the greatest effect on the commercial value of the molded products. For, by the presence of rubber particles with large particle sizes, although dart impact strength and crack resistance can be improved, silver streaks will be generated for some unclarified reason and this can be by no means disregarded.
However, the present inventors have made various investigations by changing various factors concerning the compositions comprising polyphenylene ether resins and rubber-reinforced styrenic resins, and consequently the above-described problems have been completely solved by the present invention.